As is known, urea is produced industrially using processes based on the high-temperature, high-pressure reaction of carbon dioxide and ammonia to form ammonium carbamate, and on the subsequent decomposition reaction of ammonium carbamate to form urea; the resulting urea solution is then concentrated gradually to recover the non-converted reagents; and, finally, the urea is solidified in the form of granules or prills.
In a typical plant for production of urea (i.e., a urea plant), the various stages in the process are conducted in a high-pressure section, which also has a synthesis reactor where the ammonia reacts with the carbon dioxide, a medium-pressure section, and a low-pressure section, with respective decomposers configured to decompose ammonium carbamate into urea.
Because the carbon dioxide-ammonia reaction, mainly due to the intervening ammonium carbamate, severely corrodes the stainless steel which the urea plant component parts are normally made of, some urea production processes employ passivating oxygen. That is, a gas stream containing oxygen is injected at predetermined points along the plant to passivate the metal (typically stainless steel) surfaces of the plant to prevent or reduce corrosion.
For example, a passivating oxygen-containing gas stream is fed into the medium-pressure section decomposer.
FIG. 1 shows a simplified schematic of the bottom part of a known urea plant decomposition apparatus (decomposer) 2.
Basically, decomposer 2 extends along a vertical axis A, and comprises an outer casing 4 extending about axis A and housing a pipe bundle 5 (only shown partly) between a top chamber (not shown in FIG. 1) and a bottom chamber 6.
Pipe bundle 5 is supported on a bottom pipe plate 7 and a top pipe plate (not shown in FIG. 1).
Bottom chamber 6 is located beneath pipe plate 7 and is bounded by a container or so-called ‘holder’ 8 located beneath pipe plate 7 and extending along axis A, between a bottom end 9 and a top end 10 connected to casing 4.
Holder 8 substantially flares upwards and comprises in particular a bottom catch portion 11 closed underneath by a bottom (e.g., concave) wall 12 and having a substantially cylindrical lateral wall 13 around axis A; and a funnel-shaped, upward-flaring top diffusion portion 14 having a truncated-cone-shaped lateral wall 15 around axis A.
Holder 8 has a liquid-phase outlet 16 (for the liquid phase which collects in catch portion 11 at the bottom of holder 8) located close to bottom wall 12 and through lateral wall 13; and a gaseous-phase inlet 17 located through lateral wall 13 just above the level of the liquid collected in catch portion 11 and just below the start of diffusion portion 14. Inlet 17 projects from lateral wall 13 into chamber 6, and has an upward-sloping open free end.
Inlet 17 is where the passivating gas stream is fed into decomposer 2.
Despite the passivating gas stream, however, significant corrosion is still observed, especially in the medium-pressure section decomposer.
Moreover, known decomposers, and more specifically the gas stream inlets employed in them, also pose problems in connection with the configuration of the inlets.
More specifically, as configured, certain known inlets enable the corrosive urea/carbamate mixture percolating downwards (from the pipe bundle) to enter and settle and so potentially deteriorate the material the inlet is made of.
Because of the location of the inlets—usually just above the level of the liquid accumulated at the bottom of the decomposer—an increase in the liquid level (e.g., caused by excess production), may partly or completely submerge the inlet, thus preventing the inlet from operating properly. And since known inlets have no provision for drainage, any liquid inside them cannot be removed. In other words, known decomposers of the above type are not without certain drawbacks.